1. Field of the Invention
The present invention relates to an internal machining apparatus for a hollow workpiece which spherically machines an interior spherical surface of a hollow workpiece such as differential case.
2. Discussion of the Related Art
For example, in an automotive differential case for housing a differential gear mechanism, it is required to precisely machine an interior spherical surface which contacts respective back surfaces of a pair of pinion gears meshing with a pair of side gears. Particularly, in an integral-type differential case which takes a hollow interior shape, because it is difficult to insert a cutter attached on a tool spindle from the outside of the differential case and to machine the interior spherical surface, it is practiced as described in, for example, JP2006-272468 A to machine interior flat surfaces and an interior spherical surface by successively inserting, one at a time, a plurality of machining units each with a cutting blade, by non-rotatably holding each of these machining units by a pair of unit holding arbors, and by moving the cutting blade in predetermined directions through rotation of a rotational drive member passing through one of the unit holding arbors.
That is, in the internal machining apparatus described in the aforementioned Japanese published application, where the interior spherical surface is to be machined, the cutting blade is moved along a spherical surface by a two-axis simultaneous control of axial-direction moving means for axially moving a machining unit by the cooperation of the pair of unit holding arbors and radial-direction moving means for radially moving the cutting blade held on the machining unit by the rotation of the rotational drive member passing through one of the unit holding arbors.
The internal machining apparatus described in the aforementioned Japanese published application features that the interior flat surfaces and the interior spherical surface can be machined on a single machining apparatus through exchanges of the machining units. However, by the two-axis simultaneous control of the axial-direction moving means for axially moving the machining unit and the radial-direction moving means for radially moving the cutting blade held on the machining unit, the cutting blade is moved along a spherical surface to spherically machine the interior spherical surface of the differential case. Thus, errors in locus arise when quadrants are switched to cause each axis to reverse the direction of motion, and also arise due to the adverse influence of friction force which results from the fact that the movement of the cutting blade depends on a motion conversion performed by an engaging hole and a cam follower, and therefore, a problem arises in that the application is difficult to workpieces on which the machining accuracy (e.g., sphericity) of the spherical surface is required to be strict.